McCreery Richard, Dieringer Jon, Solak Ali Osman, Snyder Brian, Nowak Aletha M, McGovern William R, DuVall Stacy
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA.
J Am Chem Soc. 2003 Sep 3;125(35):10748-58. doi: 10.1021/ja0362196.
Molecular junctions were fabricated consisting of a 3.7 nm thick layer of nitroazobenzene (NAB) molecules between a pyrolyzed photoresist substrate (PPF) and a titanium top contact which was protected from oxidation by a layer of gold. Raman spectroscopy, XPS, and AFM revealed that the NAB layer was 2-3 molecules thick and was bonded to the two conducting contacts by C-C and N-Ti covalent bonds. The current/voltage behavior of the PPF/NAB(3.7)/Ti junctions showed strong and reproducible rectification, with the current at +2 V exceeding that at -2 V by a factor of 600. The observed current density at +3 V was 0.71 A/cm(2), or about 10(5) e(-)/s/molecule. The i/V response was strongly dependent on temperature and scan rate, with the rectification ratio decreasing for lower temperature and faster scans. Junction conductivity increased with time over several seconds at room temperature in response to positive voltage pulses, with the rate of increase larger for more positive potentials. Voltage pulses to positive potentials and back to zero volts revealed that electrons are injected from the Ti to the NAB, to the extent of about 0.1-1 e(-)/molecule for a +3 V pulse. These electrons cause an activated transition of the NAB into a more conductive quinoid state, which in turn causes an increase in conductivity. The transition to the quinoid state involves nuclear rearrangement which occurs on a submillisecond to several second time scale, depending on the voltage applied. The quinoid state is stable as long as the applied electric field is present, but reverts back to NAB within several minutes after the field is relaxed. The results are interpreted in terms of a thermally activated, potential dependent electron transfer into the 3.7 nm NAB layer, which brings about a conductivity increase of several orders of magnitude.
制备了分子结，其由热解光刻胶衬底（PPF）和顶部钛接触之间的一层3.7纳米厚的硝基偶氮苯（NAB）分子组成，顶部钛接触由一层金保护以防氧化。拉曼光谱、X射线光电子能谱（XPS）和原子力显微镜（AFM）表明，NAB层为2 - 3个分子厚，并通过C - C和N - Ti共价键与两个导电接触相连。PPF/NAB(3.7)/Ti结的电流/电压行为表现出强烈且可重复的整流特性，在 +2 V时的电流比 -2 V时的电流大600倍。在 +3 V时观察到的电流密度为0.71 A/cm²，即约10⁵个电子/秒/分子。i/V响应强烈依赖于温度和扫描速率，对于较低温度和更快扫描，整流比降低。在室温下，响应正电压脉冲，结电导率在几秒钟内随时间增加，对于更正的电位，增加速率更大。向正电位然后回到零伏的电压脉冲表明，电子从Ti注入到NAB，对于 +3 V脉冲，注入程度约为0.1 - 1个电子/分子。这些电子导致NAB发生激活转变，进入导电性更强的醌式状态，这反过来又导致电导率增加。向醌式状态的转变涉及核重排，其发生在亚毫秒到几秒的时间尺度上，这取决于所施加的电压。只要存在所施加的电场，醌式状态就是稳定的，但在电场松弛后几分钟内会恢复为NAB。结果根据热激活的、电位依赖的电子转移到3.7纳米NAB层来解释，这导致电导率增加几个数量级。
